LUING. GKOLOGY. 



I must now take notice of two circumstances connected 

 with this clay slate, which were deferred in describing 

 the former islands, because the examples are most con- 

 spicuous here : they both relate to the fissile tendency 

 of this rock. 



Among the ordinary beds are to be seen irregular 

 roundish masses of the same substance, but entirely void 

 of fissile tendency. Being tougher and more durable, they 

 resist the action of the sea longer than the strata in which 

 they lie ; and are therefore often found protruding from 

 the rocks on the shore. Wherever they occur, the 

 laminae of the ordinary schist become incurvated round 

 them, quitting their naturally straight direction. A 

 circumstance resembling this, is by no means uncommon 

 in gneiss, where concretions of hornblende rock are 

 thus found surrounded by the laminse of that substance; 

 an instance of which was described in treating of Coll. 

 It also happens in micaceous schist, the nodule in this 

 case consisting of quartz. I must add that in the instance 

 now described, the nodule bears no marks of attrition, 

 nor any resemblance to an intruding substance previously 

 formed. I know not if it must be considered as an 

 example of the concretionary structure, or merely as 

 a mechanical arrangement of the yielding laminse of 

 the slate over a previously indurated substance. It is 

 rendered interesting, no less by its connexion with the 

 obscure subject of contortion in rocks, than by its re- 

 semblance to the equally obscure structure of those 

 schists which contain grains of quartz surrounded by 

 mica conforming to them ; but like many other facts, 

 the explanation must be referred to a future increase of 

 knowledge on those subjects. 



It happens generally in these islands, that the fissile 

 tendency of the clay slate is parallel to the beds ; their 

 boundaries or surfaces being determined by their alter- 

 nations with other substances. These alternations appear 

 to be often the only criterion by which we can distinguish 



